This invention relates to the detection of clear air turbulence, vertical windshear and wake vortices; and more particularly, to systems for alerting pilots to the presence of these hazards.
Clear air turbulence (CAT) and wake vortices present potential hazards to aircraft in flight. An aircraft passing through such phenomenon may experience an upset from steady, equilibrium flight. This upset may be severe enough to cause injury to passengers or in severe cases may cause a departure from controlled flight. CAT is a weather phenomenon that is due to vertical wind shear in the atmosphere and usually occurs in temperature inversion layers typically found in the tropopause.
Since the conditions that result in clear air turbulence are not visually apparent nor are they generally detectable by active sensors such as radar, there have been a number of attempts to detect wind shear and clear air turbulence conditions by passive detectors. In particular, attempts have been made to sense air temperature gradients, which are associated with air turbulence, by detecting the radiation emanating from the atmosphere ahead of the aircraft in the infrared and microwave spectral regions. The intensity of the detected radiation varies with the atmospheric temperatures along the line of sight of the detector. Typically these passive systems use a radiometer to measure the thermal radiation from one of the atmospheric gases such as carbon dioxide (CO2), oxygen (O2) or water vapor (H2O) to determine changes in the spatial temperature profile in front of the aircraft. Examples of such approaches based on the infrared emission of CO2 are provided in U.S. Pat. Nos. 3,475,963, 3,735,136, 3,780,293, 3,935,460, 4,266,130, 4,427,306, 4,937,447, 4,965,572, 4,965,573, 5,105,191, 5,276,326 and 5,285,070. Other approaches determine atmospheric temperature by measuring the microwave emission from O2 as described in U.S. Pat. Nos. 3,359,557, 3,380,055, 4,346,595, and 5,117,689.
Systems for measuring atmospheric temperature based on infrared emission from H2O are described in U.S. Pat. No. 4,266,130 and in the paper by Kuhn et al, "Clear Air Turbulence: Detection by Infrared Observations of Water Vapor" in Science, Vol. 196, p.1099, (1977). In addition, there have been several papers written describing these types of passive infrared systems including: S. M. Norman and N. H. Macoy, "Remote Detection of Clear Air Turbulence by Means of an Airborne Infrared System," AIAA Paper No. 65-459 presented at the AIAA Second Annual Meeting, San Francisco, Calif., Jul. 26-29, 1965; and R. W. Astheimer, "The Remote Detection of Clear Air Turbulence by Infrared Radiation" in Applied Optics Vol. 9, No. 8, p.1789 (1970). In U.S. Pat. No. 4,346,595, Gary describes a microwave radiometer for determining air temperatures in the atmosphere at ranges of about 3 km from the aircraft for the purpose of detecting the height of the tropopause and the presence of temperature inversions. He teaches that by flying the aircraft above or below the tropopause or temperature inversion layer, it is possible to avoid CAT. Since the effective range of the microwave radiometer is relatively short, the system doesn't provide sufficient warning time for the aircraft to avoid the CAT condition. The present invention has detection ranges on the order of 100 km which will allow time for the aircraft to change altitude to avoid CAT.
A number of the above systems were not successful or were only partially successful because they were based solely on the measurement of atmospheric temperature in order to predict the presence of turbulence. A more reliable indication of atmospheric turbulence can be realized by determining the Richardson number, Ri. The use of the Richardson number to determine the stability of the atmosphere is well known in meteorology (see, for example, D. Djuric, "Weather Analysis," Prentice Hall, Englewood Cliffs, N.J., 1994, p. 64). In U.S. Pat. No. 5,117,689, Gary discussed the correlation of the reciprocal of the Richardson number with the occurrence of CAT conditions. The Richardson number, Ri, contains two components: (1) the vertical lapse rate of potential temperature and (2) the wind shear which is related to the horizontal temperature gradient. A number of the prior art discussions measure the vertical temperature lapse rate. Gary used the inertial navigation system (INS) to measure the East-West and North-South components of the wind (the wind shear) along with a microwave radiometer to measure the air temperature vertical lapse rate. This information is then used to calculate the Richardson number or its reciprocal. The deficiency of the system described in this patent (U.S. Pat. No. 5,117,689) is that it determines the Richardson number at relatively close ranges (less than 3 km) and therefore does not provide advance warning of the CAT condition and that it measures the wind shear only at the aircraft.
Previous approaches for the determination of the range and probability of CAT can be summarized as follows:
U.S. Pat. No. 5,276,326 to Philpott determines turbulence as a function of temperature vs. range through the analysis of infrared radiometer signals at two or more discrete wavelengths. The temperature associated with a given range as a function of wavelength is then derived through a matrix inversion process. This transition is difficult and requires noise and error free input data to yield valid results. Gary overcomes the multiple wavelength difficulty in U.S. Pat. No. 4,346,595 by measuring effective temperature and range at a single wavelength, however no attempt is made to determine the probability of clear air turbulence using the Richardson number (Ri). In U.S. Pat. No. 5,117,689, Gary teaches the significance of the Richardson number in CAT prediction but does not suggest a method to derive Ri directly from radiometric measurements of horizontal and vertical temperature lapse rates obtained by combining azimuth and elevation scanning with the aircraft motion to produce a temperature map.
The above methods for airborne detection of clear air turbulence require the use of an aircraft sensor. Both infrared and radar sensors have been suggested for use. The practical difficulties involved with implementing these systems are several. First, the extremely small changes in temperature associated with the rising air current must be detected by those systems using infrared sensing. This task can be difficult to accomplish in thermally noisy environments or at long range. Second, such infrared systems require a clear lens to protect the infrared sensor. Real world flight conditions make the protection and maintenance of the lens such that reliable readings could be had costly and difficult. Third, those systems employing radar must have either a dedicated radar or must employ existing aircraft radars originally designed and dedicated for other purposes. Dedicated radar systems, such as LIDAR, tend to be extremely heavy which imposes fuel and capacity costs on the aircraft operator. The operator also must shoulder the additional burden of acquiring and maintaining a separate radar system. Fourth, the sensor is required to sweep out a large expanse of area in to either side of the aircraft and at various ranges in front of the aircraft. This requirement means that the sensor and the associated signal processing system must acquire and analyze a large quantity of data. Detecting the subtle changes indicative of turbulence becomes more difficult at long range. Furthermore, the bandwidth and time dedicated to the sensing activity can become onerous when the sensor is shared with other tasks, or when rapid update rates are desired.
Other solutions for avoiding invisible flight hazards such as CAT involve the use of mathematical atmospheric models. In particular, wake vortices models have been promulgated for several aircraft types. Air traffic controllers in the United States employ these models to develop separation rules such that one aircraft's vortices do not pose a hazard to others. One such model used by controllers is called AVOSS, or Aircraft Vortex Separation System. Such models do not actually detect the presence of vortices or turbulence, but merely indicated theoretical behaviors and regions of likely occurrence.